The present invention relates to an image forming apparatus and an image forming method which can be appropriately applied to a color printer, a color copying machine equipped with a two-sided image-forming mode that forms color images on the front and rear surfaces of a sheet and to a multifunctional machine having functions of the foregoing.
In recent years, use of a color printer and a color copying machine both being of a tandem type and of a color multifunctional machine including functions of the aforesaid items has become popular. In the image forming apparatus equipped with the single-sided image-forming mode of this kind, when reproducing R color, G color and B color of a color image, toner images for respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are formed on photoconductor drums for respective colors, and the toner images for respective colors formed on the photoconductor drums for respective colors are superposed on an intermediate transfer belt. The color toner images superposed on the intermediate transfer belt are transferred onto a desired sheet, and then, are subjected to fixing processing to be ejected.
Further, in an image forming apparatus equipped with a two-sided image-forming mode that forms a color image on the front and rear surfaces of a sheet, after the color image is formed on the front surface of a sheet, the sheet is reversed in terms of its front surface and rear surface in a transfer sheet reverse path, and a color image is transferred onto the rear surface of the sheet by conducting the aforesaid process again on the rear surface of the sheet, to be subjected to fixing processing to be ejected thereafter.
In relation to the image forming apparatus equipped with the two-sided image-forming mode of this kind, an image forming apparatus is disclosed by Patent Document 1. In this image forming apparatus, in the case of registration of images on the front and rear surfaces, an image detection sensor is arranged on the intermediate belt, and a first surface reading sensor is arranged in a reverse conveyance path, whereby, in the case of two-sided printing, a distance from a trailing edge of the first surface of the sheet to a reference image of the first surface image is detected by the first surface reading sensor, then, a reference image of an image for the second surface on the intermediate transfer body is detected, thus, sheet conveyance timing in the course of image forming on the second surface is controlled based on the results of the aforesaid both detections. If an apparatus is constructed in the aforesaid manner, reference images on the front surface and the rear surface can be aligned accurately.
Further, a recording apparatus coping with a two-sided image-forming mode is disclosed in Patent Document 2. In this recording apparatus, a sensor for detecting a leading edge of a sheet is arranged on the downstream side where a sheet supply path and a reverse conveyance path join, whereby,
relationship between a trailing edge of the first surface of the sheet and an image position is detected by the sensor in the case of two-sided printing, and a position of a leading edge of the sheet is detected in the case of recording on the second surface to establish a position of image recording on the second surface based on the results of the aforesaid detections. If an apparatus is constructed in the aforesaid manner, reference images on the front surface and the rear surface can be aligned accurately by the same sensor.
Incidentally, the image forming apparatus equipped with a two-sided image-forming mode relating to the conventional example has the following problems.
(1) In the image forming apparatus described in Patent Document 1, when aligning positions of reference images on the front surface and the rear surface, a reference image of an image for the first surface is detected by an image detection sensor arranged on an intermediate transfer body, and a distance from the trailing edge of the first surface of a transfer sheet to the first reference image formed on the front surface of the sheet is detected by the first surface reading sensor provided on the reverse conveyance path.
Therefore, when an edge portion of a transfer sheet after fixing is curled or is fluttered, its influence causes a form of an edge or a position of passage at the sensor to be unstable, and there is a fear that a detection error grows greater between the image detection sensor and the first surface reading sensor. Accordingly, it is difficult to align accurately a position of the image formed on the front surface of the sheet and a position of the image formed on the rear surface of the transfer sheet.
(2) In the recording apparatus described in Patent Document 2, a sensor for detecting a leading edge of a sheet is arranged on the downstream side where a sheet supply path and a reverse conveyance path join, and the sensor detects a leading edge of a transfer sheet and its reference image when conducting two-sided printing.
Accordingly, there is a fear that a detection error grows greater, in the case of detecting the leading edge of the transfer sheet and in the case of detecting the reference image. Incidentally, an optical-sensor of a reflection type (CIS) is used for the sensor having a function of two detections, and detection errors grow to the unignorable level, when accurate registration for positions on the front surface and the rear surface is desired.
Therefore, when behaviors of the leading edge of the transfer sheet are changed because of a sheet supply speed, a type of a sheet or a sensor position arrangement, a forming position for the image formed on the first surface (front surface) of the sheet disagrees with a forming position for the image formed on the second surface (rear surface) of the sheet, resulting in the problem that quality of image forming is lowered.    [Patent Document 1] Japanese Patent Publication Open to Public Inspection No. 2004-279749 (4th-5th pages, FIG. 1)    [Patent Document 2] Japanese Patent Publication Open to Public Inspection No. 11-237768 (2nd page, FIG. 1)